Liquid crystal display devices are electronic devices, which use variations in the transmissivity of liquid crystals according to voltage applied thereto to convert electrical information generated from various devices into visual information.
Liquid crystal display devices, which are small, lightweight, and economical in terms of power consumption, have largely replaced cathode ray tubes, and are in use in various data processing devices.
In such a liquid crystal display device, voltages are applied to a liquid crystal material to change specific molecular arrangements thereof. According to changes of the molecular arrangements, optical characteristics of the liquid crystal material also change, thereby displaying an image. The optical characteristics of the liquid crystal material include birefringence, optical rotatory power, dichroism, and light scattering.
As liquid crystal displays do not themselves produce light, they require a source of illumination in order to produce a visible image. To this end, backlight units may be used therefor.
Backlight units may be classified into edge-type backlight units and direct-type backlight units according to a position of a light emitting device therein. Edge-type backlight units include a light emitting device at a side of a light guide panel that guides light emitted from the lamp. Edge-type backlight units are used in small liquid crystal display devices such as the monitors of desktop and notebook computers, and provide even lighting and good durability. Moreover, the edge-type backlight units make possible the slimming of a device including the edge-type backlight unit. Meanwhile, the direct-type backlight units are used in 20-inch or larger display devices. Such a direct-type backlight unit includes lamps arrayed under a liquid crystal panel to directly illuminate the liquid crystal panel.
Linear light sources such as a cold cathode fluorescent lamp (CCFL) were previously widely used as light emitting devices in backlight units, but recently, linear light sources have increasingly been replaced with light emitting diodes (LEDs), which are slim, lightweight, economical in terms of power consumption, have excellent color reproducibility, and are environmentally sound.
A backlight unit may include a combination of optical films to diffuse or collect light emitted from a light source and improve the brightness and unevenness of the lighting thereof.
Brightness and viewing angle are important factors in evaluating image display devices such as liquid crystal display devices, and are largely determined by the performance of an optical sheet constituting a backlight unit. In recent years, hemisphere-shaped microlens array sheets have been widely used, but they have a limitation in improving brightness, and the brightness must be sacrificed in order to improve a viewing angle thereof.
Today, active research and development is being carried out into creating slim and lightweight backlight units. In particular, microlens array sheets are required to have optical performances such as high levels of brightness and wide viewing angles.